The present invention relates to a method for monitoring and/or controlling the average cell size of a plastic foam structure continuously emerging from a production extrusion source. The invention also relates to an apparatus for monitoring and/or controlling the average cell size of such a foam structure.
By continuously monitoring this foam characteristic, i.e., cell size, the variation of which has a pronounced affect upon the physical properties of the foam, prompt processing and/or material feed (e.g., blowing agent, nucleating agent, etc.) adjustments may be made to bring the cell size back into a specified range in the event that it deviates therefrom.
It is known in the prior art that during the extrusion of polymer foam, cell size is the structural parameter which can be modified most readily and, furthermore, has a pronounced affect on the foam properties. These properties include compressive strength, tensile strength, elongation at break, tear strength and thermal insulating values. Cell size can be varied by orders of magnitude in a full scale production process by manipulating the necessary process parameters, nucleating agent, melt temperature, and so forth. The quantification of cell size is, under normal circumstances, tedious and time consuming. Normally a thin section of foam is prepared using a suitable cutting device, microtome, or its equivalent. The sample is then examined under a microscope or an image of the cell structure is projected onto a frosted glass plate in magnified form. An attempt is made to characterize the cell size by measuring the mean diameter of the cell or counting the number of cells contained in a given area. Obviously these prior art methods do not lend themselves to reproducible or standardized measurement. They do not necessarily reflect the average cell size of the same under investigation, since distribution of cell sizes occur in most foam polymer systems and single point measurements cannot always be translated into an average measurement. In addition, these kinds of cell size quantification can be costly in a production situation. Poor quality and/or "off-specification" material can be manufactured for long periods of time while an out-of-line cell size measurement is being made. Accordingly, it is an object of the present invention to provide a more accurate method and system for determining the cell size of polymeric foams. Further, the present invention provides an automatic process for determining and recording cell size substantially, continuously or at predetermined time intervals.